The present disclosure relates to centrifugal compressors, such as used in turbochargers, and more particularly relates to centrifugal compressors in which the effective inlet area or diameter can be adjusted for different operating conditions.
An exhaust gas-driven turbocharger is a device used in conjunction with an internal combustion engine for increasing the power output of the engine by compressing the air that is delivered to the air intake of the engine to be mixed with fuel and burned in the engine. A turbocharger comprises a compressor wheel mounted on one end of a shaft in a compressor housing and a turbine wheel mounted on the other end of the shaft in a turbine housing. Typically the turbine housing is formed separately from the compressor housing, and there is yet another center housing connected between the turbine and compressor housings for containing bearings for the shaft. The turbine housing defines a generally annular chamber that surrounds the turbine wheel and that receives exhaust gas from an engine. The turbine assembly includes a nozzle that leads from the chamber into the turbine wheel. The exhaust gas flows from the chamber through the nozzle to the turbine wheel and the turbine wheel is driven by the exhaust gas. The turbine thus extracts power from the exhaust gas and drives the compressor. The compressor receives ambient air through an inlet of the compressor housing and the air is compressed by the compressor wheel and is then discharged from the housing to the engine air intake.
Turbochargers typically employ a compressor wheel of the centrifugal (also known as “radial”) type because centrifugal compressors can achieve relatively high pressure ratios in a compact arrangement. Intake air for the compressor is received in a generally axial direction at an inducer portion of the centrifugal compressor wheel and is discharged in a generally radial direction at an exducer portion of the wheel. The compressed air from the wheel is delivered to a volute, and from the volute the air is supplied to the intake of an internal combustion engine.
The operating range of the compressor is an important aspect of the overall performance of the turbocharger. The operating range is generally delimited by a surge line and a choke line on an operating map for the compressor. The compressor map is typically presented as pressure ratio (discharge pressure Pout divided by inlet pressure Pin) on the vertical axis, versus corrected mass flow rate on the horizontal axis. The choke line on the compressor map is located at high flow rates and represents the locus of maximum mass-flow-rate points over a range of pressure ratios; that is, for a given point on the choke line, it is not possible to increase the flow rate while maintaining the same pressure ratio because a choked-flow condition occurs in the compressor.
The surge line is located at low flow rates and represents the locus of minimum mass-flow-rate points without surge, over a range of pressure ratios; that is, for a given point on the surge line, reducing the flow rate without changing the pressure ratio, or increasing the pressure ratio without changing the flow rate, would lead to surge occurring. Surge is a flow instability that typically occurs when the compressor blade incidence angles become so large that substantial flow separation arises on the compressor blades. Pressure fluctuation and flow reversal can happen during surge.
In a turbocharger for an internal combustion engine, compressor surge may occur when the engine is operating at high load or torque and low engine speed, or when the engine is operating at a low speed and there is a high level of exhaust gas recirculation (EGR). Surge can also arise when an engine is suddenly decelerated from a high-speed condition. Expanding the surge-free operation range of a compressor to lower flow rates is a goal often sought in compressor design.
Choked flow can also occur in the compressor at high flow rates. It would be desirable to have a means for shifting the choke line to higher flow rates when operating conditions require it.